As a related art, there is a substrate for high-frequency device mounting fabricated as follows. The substrate is formed by stacking an upper insulating layer over a lower insulating layer with the intermediary of a ground layer. Through-holes are opened in the ground layer. The inner diameter of the through-holes is set to a small diameter to lower the characteristic impedance of conductor line parts loosely inserted in the through-holes. The lower insulating layer is formed with a large thickness to raise the characteristic impedance of conductor line parts formed to vertically penetrate the lower insulating layer. Furthermore, matching of the characteristic impedance of conductor lines to a predetermined value is carried out. Signal lines are made on the upper insulating layer. The signal lines are coupled to the conductor lines and constitute a microstrip line with the ground layer.
In the related-art substrate for high-frequency device mounting, adjustment of the characteristic impedance is not carried out at the coupling parts between the conductor lines and the signal lines. For example, if the frequency is as high as 100 GHz to several hundreds of GHz or higher, the importance of adjustment of the characteristic impedance at the coupling parts is high. If the characteristic impedance is not set to the desired value, the operation characteristics of the electronic device deteriorate.
The following is a reference document. [Document 1] Japanese Laid-open Patent Publication No. 08-078797.